1. Field of the Invention
The present invention relates generally to transducers and, more particularly, to an interface circuit and smart sensor module for interfacing with a variety of analog transducers.
2. Discussion of the Related Art
Resistive bridge and other types of voltage output sensors are commonly used to measure acceleration, pressure and other types of conditions; however, imperfect matching of resistive components of the bridge as well as pre-existing stress on the sensor surface tend to produce variations of the initial offset voltage between sensors. In addition, bridge offset generally changes with the operating temperature. Traditional sensor design relied on analog circuitry to calibrate the initial zero offset and sensitivity of the sensor, as well as to compensate for nonlinear temperature errors of offset and sensitivity. Sensor-specific compensation and calibration data were stored using memory components including potentiometers and discrete and laser-trimmed resistors. More recently, digital signal processing has permitted sensor-specific data to be stored in programmable memory such as random access memory (RAM) and electrically programmable read only memory (EPROM).
Currently, there are two basic digital signal conditioning approaches for smart sensors providing an analog output signal. In the first approach, the sensor analog output signal is converted to a digital signal and signal conditioning is carried out in the digital domain, with the processed digital signal either being used directly by the microprocessor or converted back to the analog domain for use by other devices. A disadvantage of this type of approach is that sensor output must be digitized twice in addition to being conditioned in the digital domain. This can reduce signal resolution, introduce quantization errors and increase response time. The digital signal processing approach also requires a relatively powerful microprocessor capable of performing all of the calculations needed to condition the signal, thereby increasing the size and cost of the device as well as power consumption. In the second approach, sensor errors are corrected and normalized based on digitally programmable gain and offset adjustment of the input amplifier. While this approach permits the use of a less powerful microprocessor and concomitant reductions in size, cost and power consumption, the ability to accommodate different sensors is typically limited by the fixed component values of the system. In addition, the accuracy and range of correction is somewhat less than that typically possible with the digital signal processing approach due to practical limitations relating to the size of the digital-to-analog converters.